Giant magnetoresistive sensor with a high resistivity free layer

ABSTRACT

A spin valve (SV) magnetoresistive sensor is provided having an AP-pinned layer, a laminated ferromagnetic free layer and a non-magnetic electrically conductive spacer layer sandwiched between the AP-pinned layer and the free layer. The AP-pinned layer comprises first and second ferromagnetic layers separated by an antiparallel coupling (APC) layer. The laminated free layer comprises a third ferromagnetic layer of Co—Fe adjacent to the spacer layer and a fourth ferromagnetic layer of Co—Fe—Hf—O. The Co—Fe—Hf—O material of the fourth ferromagnetic layer has high resistivity resulting in reduced sense current shunting by the free layer. In addition, the metal oxide material of the fourth ferromagnetic layer is known to cause specular scattering of electrons. The reduced sense current shunting and the specular scattering of electrons both contribute to improving the GMR coefficient of the SV sensor.

CROSS REFERENCE TO RELATED APPLICATION

U.S. patent application Ser. No. 09/630,329, entitled GIANTMAGNETORESISTIVE SENSOR WITH AN AP-COUPLED LOW H_(k) FREE LAYER, wasfiled on the same day, owned by a common assignee and having the sameinventor as the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to giant magnetoresistive (GMR)sensors for reading information signals from a magnetic medium and, inparticular, to a spin valve sensor having a free layer of highresistivity, soft magnetic material to improve the GMR coefficient, andto magnetic storage systems that incorporate such sensors.

2. Description of Related Art

Computers often include auxiliary memory storage devices having media onwhich data can be written and from which data can be read for later use.A direct access storage device (disk drive) incorporating rotatingmagnetic disks is commonly used for storing data in magnetic form on thedisk surfaces. Data is recorded on concentric, radially spaced tracks onthe disk surfaces. Magnetic heads including read sensors are then usedto read data from the tracks on the disk surfaces.

In high capacity disk drives, magnetoresistive (MR) read sensors,commonly referred to as MR sensors, are the prevailing read sensorsbecause of their capability to read data from a surface of a disk atgreater track and linear densities than thin film inductive heads. An MRsensor detects a magnetic field through the change in the resistance ofits MR sensing layer (also referred to as an “MR element”) as a functionof the strength and direction of the magnetic flux being sensed by theMR layer.

The conventional MR sensor operates on the basis of the anisotropicmagnetoresistive (AMR) effect in which an MR element resistance variesas the square of the cosine of the angle between the magnetization inthe MR element and the direction of sense current flowing through the MRelement. Recorded data can be read from a magnetic medium because theexternal magnetic field from the recorded magnetic medium (the signalfield) causes a change in the direction of magnetization in the MRelement, which in turn causes a change in resistance in the MR elementand a corresponding change in the sensed current or voltage.

Another type of MR sensor is the giant magnetoresistance (GMR) sensormanifesting the GMR effect. In GMR sensors, the resistance of the MRsensing layer varies as a function of the spin-dependent transmission ofthe conduction electrons between magnetic layers separated by anon-magnetic layer (spacer) and the accompanying spin-dependentscattering which takes place at the interface of the magnetic andnon-magnetic layers and within the magnetic layers.

GMR sensors using only two layers of ferromagnetic material (e.g.,Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) aregenerally referred to as spin valve (SV) sensors manifesting the SVeffect. FIG. 1 shows a prior art SV sensor 100 comprising end regions104 and 106 separated by a central region 102. A first ferromagneticlayer, referred to as a pinned layer 120, has its magnetizationtypically fixed (pinned) by exchange coupling with an antiferromagnetic(AFM) layer 125. The magnetization of a second ferromagnetic layer,referred to as a free layer 110, is not fixed and is free to rotate inresponse to the magnetic field from the recorded magnetic medium (thesignal field). The free layer 110 is separated from the pinned layer 120by a non-magnetic, electrically conducting spacer layer 115. Leads 140and 145 formed in the end regions 104 and 106, respectively, provideelectrical connections for sensing the resistance of SV sensor 100.IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporatedherein by reference, discloses a SV sensor operating on the basis of theGMR effect.

Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. InAP-pinned SV sensors, the pinned layer is a laminated structure of twoferromagnetic layers separated by a non-magnetic coupling layer suchthat the magnetizations of the two ferromagnetic layers are stronglycoupled together antiferromagnetically in an antiparallel orientation.The AP-pinned SV sensor provides improved exchange coupling of theantiferromagnetic (AFM) layer to the laminated pinned layer structurethan is achieved with the pinned layer structure of the SV sensor ofFIG. 1. This improved exchange coupling increases the stability of theAP-pinned SV sensor at high temperatures which allows the use ofcorrosion resistant and electrically insulating antiferromagneticmaterials such as NiO for the AFM layer.

Referring to FIG. 2, an AP-pinned SV sensor 200 comprises a free layer210 separated from a laminated AP-pinned layer structure 220 by anonmagnetic, electrically-conducting spacer layer 215. The magnetizationof the laminated AP-pinned layer structure 220 is fixed by an AFM layer230. The laminated AP-pinned layer structure 220 comprises a firstferromagnetic layer 226 and a second ferromagnetic layer 222 separatedby an antiparallel coupling (APC) layer 224 of nonmagnetic material(usually ruthenium (Ru)). The two ferromagnetic layers 226, 222 (FM1 andFM2) in the laminated AP-pinned layer structure 220 have theirmagnetization directions oriented antiparallel, as indicated by thearrows 227, 223 (arrows pointing out of and into the plane of the paperrespectively).

As of high storage capacity disk drives, it is increasingly important toincrease the GMR coefficient of SV sensors in order to improve thesensitivity and signal-to-noise characteristics of the signal readbacksystem. Sense current shunting around the spacer layer and the pinnedlayer and spacer layer interfaces with the spacer layer results inreduces GMR coefficient since most of the spin dependent scatteringgiving rise to the GMR effect occurs in this region. The free layer ofSV sensors usually consists of Co—Fe and Ni—Fe layers. The Co—Fe is usedto obtain a high GMR coefficient, and the Ni—Fe is added to achieve afree layer with soft magnetic properties. However, the Ni—Fe has a lowelectrical resistivity which contributes to sense current shuntingresulting in a decrease of the GMR coefficient.

Therefore, there is a need for an improved free layer to reduce sensecurrent shunting and to increase the GMR coefficient of a spin valvesensor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to disclose a spinvalve sensor having a free layer of high electrical resistivity, softferromagnetic material.

It is another object of the present invention to disclose a spin valvesensor having an improved GMR coefficient due to reduced currentshunting by the ferromagnetic free layer.

It is a further object of the present invention to disclose a spin valvesensor having a laminated free layer comprising a third ferromagneticlayer of Co—Fe and a fourth ferromagnetic layer of Co—Fe—Hf—O.

In accordance with the principles of the present invention, there isdisclosed a spin valve (SV) sensor having an AP-pinned layer, alaminated ferromagnetic free layer and a non-magnetic electricallyconductive spacer layer sandwiched between the AP-pinned layer and thefree layer. The AP-pinned layer comprises first and second ferromagneticlayers separated by an antiparallel coupling (APC) layer. The laminatedfree layer comprises a third ferromagnetic layer of Co—Fe adjacent tothe spacer layer and a fourth ferromagnetic layer of Co—Fe—Hf—O. TheCo—Fe—Hf—O material of the fourth ferromagnetic layer has highresistivity resulting in reduced sense current shunting by the freelayer. In addition, the metal oxide material of the fourth ferromagneticlayer is known to cause specular scattering of electrons. The reducedsense current shunting and the specular scattering of electrons bothcontribute to improving the GMR coefficient of the SV sensor.

The above, as well as additional objects, features and advantages of thepresent invention will become apparent in the following detailed writtendescription.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings. In the following drawings, like referencenumerals designate like or similar parts throughout the drawings.

FIG. 1 is an air bearing surface view, not to scale, of a prior art SVsensor;

FIG. 2 is an air bearing surface view , not to scale, of a prior artAP-pinned SV sensor;

FIG. 3 is a simplified diagram of a magnetic recording disk drive systemusing the spin valve transistor sensor of the present invention;

FIG. 4 is a vertical cross-section view, not to scale, of a “piggyback”read/write magnetic head;

FIG. 5 is a vertical cross-section view, not to scale, of a “merged”read/write magnetic head; and

FIG. 6 is an air bearing surface view, not to scale, of an embodiment ofthe spin valve sensor of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description is the best embodiment presently contemplatedfor carrying out the present invention. This description is made for thepurpose of illustrating the general principles of the present inventionand is not meant to limit the inventive concepts claimed herein.

Referring now to FIG. 3, there is shown a disk drive 300 embodying thepresent invention. As shown in FIG. 3, at least one rotatable magneticdisk 312 is supported on a spindle 314 and rotated by a disk drive motor318. The magnetic recording media on each disk is in the form of anannular pattern of concentric data tracks (not shown) on the disk 312.

At least one slider 313 is positioned on the disk 312, each slider 313supporting one or more magnetic read/write heads 321 where the head 321incorporates the SV sensor of the present invention. As the disksrotate, the slider 313 is moved radially in and out over the disksurface 322 so that the heads 321 may access different portions of thedisk where desired data is recorded. Each slider 313 is attached to anactuator arm 319 by means of a suspension 315. The suspension 315provides a slight spring force which biases the slider 313 against thedisk surface 322. Each actuator arm 319 is attached to an actuator 327.The actuator as shown in FIG. 3 may be a voice coil motor (VCM). The VCMcomprises a coil movable within a fixed magnetic field, the directionand speed of the coil movements being controlled by the motor currentsignals supplied by a controller 329.

During operation of the disk storage system, the rotation of the disk312 generates an air bearing between the slider 313 (the surface of theslider 313 which includes the head 321 and faces the surface of the disk312 is referred to as an air bearing surface (ABS)) and the disk surface322 which exerts an upward force or lift on the slider. The air bearingthus counter-balances the slight spring force of the suspension 315 andsupports the slider 313 off and slightly above the disk surface by asmall, substantially constant spacing during normal operation.

The various components of the disk storage system are controlled inoperation by control signals generated by the control unit 329, such asaccess control signals and internal clock signals. Typically, thecontrol unit 329 comprises logic control circuits, storage chips and amicroprocessor. The control unit 329 generates control signals tocontrol various system operations such as drive motor control signals online 323 and head position and seek control signals on line 328. Thecontrol signals on line 328 provide the desired current profiles tooptimally move and position the slider 313 to the desired data track onthe disk 312. Read and write signals are communicated to and from theread/write heads 321 by means of the recording channel 325. Recordingchannel 325 may be a partial response maximum likelihood (PMRL) channelor a peak detect channel. The design and implementation of both channelsare well known in the art and to persons skilled in the art. In thepreferred embodiment, recording channel 325 is a PMRL channel.

The above description of a typical magnetic disk storage system, and theaccompanying illustration of FIG. 3 are for representation purposesonly. It should be apparent that disk storage systems may contain alarge number of disks and actuator arms, and each actuator arm maysupport a number of sliders.

FIG. 4 is a side cross-sectional elevation view of a “piggyback”magnetic read/write head 400, which includes a write head portion 402and a read head portion 404, the read head portion employing a spinvalve (SV) sensor 406 according to the present invention. The SV sensor406 is sandwiched between nonmagnetic insulative first and second readgap layers 408 and 410, and the read gap layers are sandwiched betweenferromagnetic first and second shield layers 412 and 414. In response toexternal magnetic fields, the resistance of the SV sensor 406 changes. Asense current I_(s) conducted through the sensor causes these resistancechanges to be manifested as potential changes. These potential changesare then processed as readback signals by the processing circuitry ofthe data recording channel 346 shown in FIG. 3.

The write head portion 402 of the magnetic read/write head 400 includesa coil layer 416 sandwiched between first and second insulation layers418 and 420. A third insulation layer 522 may be employed forplanarizing the head to eliminate ripples in the second insulation layer420 caused by the coil layer 416. The first, second and third insulationlayers are referred to in the art as an insulation stack. The coil layer416 and the first, second and third insulation layers 418, 420 and 422are sandwiched between first and second pole piece layers 424 and 426.The first and second pole piece layers 424 and 426 are magneticallycoupled at a back gap 428 and have first and second pole tips 430 and432 which are separated by a write gap layer 434 at the ABS 440. Aninsulation layer 436 is located between the second shield layer 414 andthe first pole piece layer 424. Since the second shield layer 414 andthe first pole piece layer 424 are separate layers this read/write headis known as a “piggyback” head.

FIG. 5 is the same as FIG. 4 except the second shield layer 514 and thefirst pole piece layer 524 are a common layer. This type of read/writehead is known as a “merged” head 500. The insulation layer 436 of thepiggyback head in FIG. 4 is omitted in the merged head 500 of FIG. 5.

FIG. 6 shows an air bearing surface (ABS) view, not to scale, of anantiparallel (AP)-pinned spin valve (SV) sensor 600 according to anembodiment of the present invention. The SV sensor 600 comprises endregions 602 and 604 separated from each other by a central region 606.The substrate 608 can be any suitable substance, including glass,semiconductor material, or a ceramic material, such as alumina (Al₂O₃).The seed layer 610 is a layer deposited to modify the crystallographictexture or grain size of the subsequent layers, and may not be neededdepending on the material of the subsequent layer. For the embodiment ofthe SV sensor 600, the seed layer 610 comprises a trilayer seed layerstructure deposited on the substrate. First, second and third sublayers612, 614 and 616 are sequentially deposited on the substrate 608. Anantiferromagnetic (AFM) layer 620 is deposited over the third sublayer612 to the thickness at which the desired exchange properties areachieved, typically 100-500 ÅA. A laminated AP-pinned layer 622 isformed on the AFM layer 620 in the central region 606. The AP-pinnedlayer 622 comprises a first ferromagnetic layer (FM1) 624, a secondferromagnetic layer (FM2) 628 and an antiparallel coupling (APC) layer626 disposed between the FM1 layer 624 and the FM2 layer 628. The APClayer is formed of a nonmagnetic material, preferably ruthenium (Ru),that allows the FM1 layer 624 and the FM2 layer 628 to be stronglycoupled together antiferromagnetically. A laminated free layer (freeferromagnetic layer) 632 including a third ferromagnetic layer (FM3) 634and a fourth ferromagnetic layer (FM4) 636 is separated from theAP-pinned layer 622 by a nonmagnetic electrically conducting spacerlayer 630. The magnetization of the free layer 632 is preferablyparallel to the ABS in the absence of an external field as indicated bythe arrow 633. A cap layer 638, formed on the free layer 632, completesthe central region 606 of the SV sensor 600.

In the present embodiment, the cap layer 638 is formed of tantalum (Ta).Alternatively, the cap layer 638 may be a bilayer cap layer formed of afirst sublayer of copper (Cu) formed on the free layer 632 and a secondsublayer of of tantalum (Ta) formed on the first sublayer of Cu. Thepresence of a Cu layer above the free layer is known to enhance themagnetoresistance of the SV sensor through a spin filter effect.

The SV sensor 600 further comprises bias layers 640 and 642 formed onthe end regions 602 and 604, respectively, for providing a longitudinalbias field to the free layer 632 to ensure a single magnetic domainstate in the free layer. Lead layers 644 and 646 are also deposited onthe end regions 602 and 604, respectively, to provide electricalconnections for the flow of a sensing current I_(s) from a currentsource 650 to the SV sensor 600. A signal detector 660 which iselectrically connected to leads 644 and 646 senses the change inresistance due to changes induced in the free layer 632 by the externalmagnetic field (e.g., field generated by a data bit stored on a disk).The external magnetic field acts to rotate the direction ofmagnetization of the free layer 632 relative to the direction ofmagnetization of the pinned layer 622 which is preferably pinnedperpendicular to the ABS. The signal detector 660 preferably comprises apartial response maximum likelihood (PRML) recording channel forprocessing the signal detected by SV sensor 600. Alternatively, a peakdetect channel or a maximum likelihood channel (e.g., 1,7 ML) may beused. The design and implementation of the aforementioned channels areknown to those skilled in the art. The signal detector 660 also includesother supporting circuitries such as a preamplifier (electrically placedbetween the sensor and the channel) for conditioning the sensedresistance changes as is known to those skilled in the art.

The SV sensor 600 is fabricated in a magnetron sputtering or an ion beamsputtering system to sequentially deposit the multilayer structure shownin FIG. 6. The sputter deposition process is carried out in the presenceof a longitudinal magnetic field of about 40 Oe. The seed layer 610 isformed on the substrate 608 by sequentially depositing the firstsublayer 612 of Al₂O₃ having a thickness of about 30 Å, the secondsublayer 614 of NiMnO having a thickness of about 30 Å and the thirdsublayer 616 of tantalum (Ta) having a thickness of about 30 Å. The AFMlayer 620 formed of Pt—Mn having a thickness of about 200 Å is depositedon the third sublayer 616 of the seed layer 610.

The AP-pinned layer 622, the spacer layer 630, the laminated free layer632 and the cap layer 638 are sequentially deposited on the AFM layer620 in the central region 606. The FM1 layer 624 of Co—Fe having athickness of about 17 Å is deposited on the AFM layer 620. The APC layer626 of ruthenium having a thickness of about 8 Å is deposited on the FM1layer 624. The FM2 layer 628 of Co—Fe having a thickness of about 26 Åis deposited on the APC layer 626.

The nonmagnetic conducting spacer layer 630 is formed of copper (Cu)having a thickness of about 21 Å deposited on the FM2 layer 628.Alternatively, the spacer layer 630 may be formed of silver (Ag), gold(Au) or of alloys of Cu, Ag and Au. The laminated free layer 632comprises the FM3 layer 634 of Co—Fe having a thickness in the range of10-20 Å, preferably 15 Å, deposited on the spacer layer 630 and the FM4layer 636 of Co—Fe—Hf—O having a thickness in the range of 10-20 Å,preferably 15 Å, deposited on the FM3 layer 634. The cap layer 638 isformed of Ta having a thickness of about 50 Å deposited on the FM4 layer638 of the free layer 632.

After the deposition of the central portion 606 is completed, the sensoris annealed in the presence of a magnetic field of about 800 Oe orientedin the transverse direction to the ABS and is then cooled while still inthe magnetic field to set the exchange coupling of the AFM layer 620with the laminated AP-pinned layer 622 transverse to the ABS. The FM1layer 624 has a surface which interfaces with a surface of the AFM layer620 so that the AFM layer pins the magnetic moment 625 (represented inFIG. 6 by the tail of an arrow pointing into the plane of the paper) ofthe FM1 layer 624 in a direction perpendicular to and away from the ABS.The magnetization of the FM1 layer 624 is pinned in this direction byexchange coupling with the AFM layer 620. The APC layer 626 is very thin(about 8 Å) which allows an antiferromagnetic exchange coupling betweenthe FM1 layer 624 and the FM2 layer 628. Accordingly, the magnetization629 (represented by the head of an arrow pointing out of the plane ofthe paper) of the FM2 layer 628 is directed in an opposite direction tothe magnetization 625 of the FM1 layer 624, namely perpendicular to andtowards the ABS. Alternatively, the magnetization 625 of the FM1 layer624 may be set in an opposite direction (perpendicular to and away fromthe ABS) so that the magnetization 625 points out of the plane of thepaper. The magnetization 629 of the FM2 layer 628 will then point intothe plane of the paper due to the antiparallel coupling across the APClayer 626.

A novel feature of the present invention is the use of Co—Fe—Hf—c toform the FM4 layer 636 of the laminated free layer 632. The FM4 layer636 is a sublayer of the free layer 632 separated from the spacer layer630 by the FM3 layer 634 of Co—Fe. The ferromagnetic Co—Fe—Hf—O materialis known to have a a very high electrical resistivity (>400 μohm-cm) andto possess soft magnetic properties (coercivity H_(c)<5 Oe, andintrinsic uniaxial anisotropy H_(k)<10 Oe). The soft magnetic propertiesare important in allowing the free layer 632 to rotate freely inresponse to an signal magnetic field. The high resistance of the FM4layer 636 reduces sense current flow through this sublayer of the freelayer 632 resulting in a increase of the sense current flow in thespacer layer 630 and the ferromagnetic layers interfacing the spacerlayer where the spin dependent scattering processes that result in theGMR effect are most effective. In addition to high resistivity, theCo—Fe—Hf—O material is a metal oxide known to cause specular reflectionof electrons. Electrons are specularly reflected by the metal oxidematerial back toward the free layer where they continue to add to theGMR effect. The combined effects of reduced sense current shunting andspecular reflection of electrons scattered into the metal oxide layerwill result in an increased GMR coefficient for the spin valve sensor600. Furthermore, the Co—Fe—Hf—o material is known to have high thermalstability. The transverse anneal process used to set the exchangecoupling of the AFM layer 620 of Pt—Mn with the AP-pinned layer 622transverse to the ABS will not cause rotation of the longitudinalorientation of the magnetic easy axis of the Co—Fe—Hf—O material of theFM4 layer, however, the magnetic easy axis of the Co—Fe material of theFM3 layer is rotated to a transverse orientation. As a result, the netintrinsic uniaxial anisotropy H_(k) of the FM3 and FM4 layers of thefree layer 632 will be reduced.

The high electrical resistivity and low uniaxial anisotropy of theCo—Fe—Hf—O material is due to a nano-grain crystalline structure. Thecomposition range of the desired material may be expressed as(Co_(a)—Fe_(b))_(x)—Hf_(y)—O_(z) in atomic percent, where 40%≦x≦70%,5%≦y≦25%, 20%≦z≦40%, 70%≦a≦95%, 5%≦b≦30%, x+y+z=100% and a+b=100%. Thepreferred composition is (Co₉₀—Fe₁₀)₆₀—Hf₁₀—O₃₀.

The FM3 layer 634 of the laminated free layer 632 is made of Co—Fehaving a composition range expressed as Co_(a)—Fe_(b) in atomic percent,where 70%≦a≦95%, 5%≦b≦30%, and a+b=100%. The preferred composition isCo₉₀—Fe₁₀.

While the present invention has been particularly shown and describedwith reference to the preferred embodiment, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit, scope and teaching of theinvention. Accordingly, the disclosed invention is to be consideredmerely as illustrative and limited in scope only as specified in theappended claims.

I claim:
 1. A spin valve (SV) magnetoresistive sensor, comprising: anantiferromagnetic (AFM) layer; a pinned layer adjacent to said AFMlayer, said AFM layer fixing a magnetization direction of said pinnedlayer; a ferromagnetic free layer comprising; a third ferromagneticlayer; and a fourth ferromagnetic layer of high electrical resistivitymaterial adjacent to said third ferromagnetic layer, wherein said fourthferromagnetic layer is made of Co—Fe—Hf—O; and a spacer layer ofnonmagnetic electrically conductive material disposed between saidpinned layer and said third ferromagnetic layer.
 2. The SVmagnetoresistive sensor as recited in claim 1 wherein said fourthferromagnetic layer is made of (Co₉₀—Fe₁₀)₆₀—Hf₁₀—O₃₀.
 3. The SVmagnetoresistive-sensor as recited in claim 1 wherein said fourthferromagnetic layer is made of (Co_(a)—Fe_(b))_(x)—Hf_(y)—O_(z) inatomic percent, where 40%≦x≦70%, 5%≦y≦25%, 20%≦z≦40%, 70%≦a≦95%,5%≦b≦30%, x+y+z=100% and a+b=100%.
 4. A spin valve (SV) magnetoresistivesensor, comprising: an antiferromagnetic (AFM) layer; an antiparallel(AP)-pinned layer adjacent to said AFM layer, said AP-pinned layercomprising; a first ferromagnetic layer adjacent to said AFM layer; asecond ferromagnetic layer; and an antiparallel coupling (APC) layerdisposed between said first ferromagnetic layer and said secondferromagnetic layer; a ferromagnetic free layer comprising; a thirdferromagnetic layer; and a fourth ferromagnetic layer of highelectrical. resistivity material adjacent to said third ferromagneticlayer, wherein said fourth ferromagnetic layer is made of Co—Fe—Hf—O;and a spacer layer of nonmagnetic electrically conductive materialdisposed between said second ferromagnetic layer and said thirdferromagnetic layer.
 5. The SV magnetoresistive sensor as recited inclaim 4 wherein said fourth ferromagnetic layer is made of(Co₉₀—Fe₁₀)₆₀—Hf₁₀—O₃₀.
 6. The SV magnetoresistive sensor as recited inclaim 4 wherein said fourth ferromagnetic layer is made of(Co_(a)—Fe_(b))_(x)—Hf_(y)—O_(z) in atomic percent, where 40%≦x≦70%,5%≦y≦25%, 20%≦z≦40%, 70%≦a≦95%, 5%≦b≦30%, x+y+z=100% and a+b=100%.
 7. Amagnetic read/write head comprising: a write head including: at leastone coil layer and an insulation stack, the coil layer being embedded inthe insulation stack; first and second pole piece layers connected at aback gap and having pole tips with edges forming a portion of an airbearing surface (ABS); the insulation stack being sandwiched between thefirst and second pole piece layers; and a write gap layer sandwichedbetween the pole tips of the first and second pole piece layers andforming a portion of the ABS; a read head including: a spin valve (SV)magnetoresistive sensor, comprising: an antiferromagnetic (AFM) layer;an antiparallel (AP)-pinned layer adjacent to said AFM layer, saidAP-pinned layer comprising; first ferromagnetic layer adjacent to saidAFM layer; a second ferromagnetic layer; and an antiparallel coupling(APC) layer disposed between said first ferromagnetic layer and saidsecond ferromagnetic layer; a ferromagnetic free layer comprising; athird ferromagnetic layer; and a fourth ferromagnetic layer of highelectrical resistivity material adjacent to said third ferromagneticlayer, wherein said fourth ferromagnetic layer is made of Co—Fe—Hf—O;and a spacer layer of nonmagnetic electrically conductive materialdisposed between said second ferromagnetic layer and said thirdferromagnetic layer; and an insulation layer disposed between the secondshield layer of the read head and the first pole piece layer of thewrite head.
 8. The magnetic read/write head as recited in claim 7wherein said fourth ferromagnetic layer is made of(Co₉₀—Fe₁₀)₆₀—Hf₁₀—O₃₀.
 9. The magnetic read/write head as recited inclaim 7 wherein said fourth ferromagnetic layer is made of(Co_(a)—Fe_(b))_(x)—Hf_(y)—O_(z) in atomic percent, where 40%≦x≦70%,5%≦y≦25%, 20%≦z≦40%, 70%≦a≦95%, 5%≦b≦30%, x+y+z=100% and a+b=100%.
 10. Adisk drive system comprising: a magnetic recording disk; a magneticread/write head for magnetically recording data on the magneticrecording disk and for sensing magnetically recorded data on themagnetic recording disk, said magnetic read/write head comprising: awrite head including: at least one coil layer and an insulation stack,the coil layer being embedded in the insulation stack; first and secondpole piece layers connected at a back gap and having pole tips withedges forming a portion of an air bearing surface (ABS); the insulationstack being sandwiched between the first and second pole piece layersand a write gap layer sandwiched between the pole tips of the first andsecond pole piece layers and forming a portion of the ABS; a read headincluding: a spin valve (SV) magnetoresistive sensor comprising: anantiferromagnetic (AFM) layer; an antiparallel (AP)-pinned layeradjacent to said AFM layer, said AP-pinned layer comprising;  a firstferromagnetic layer adjacent to said AFM layer;  a second ferromagneticlayer; and  an antiparallel coupling (APC) layer disposed between saidfirst ferromagnetic layer and said second ferromagnetic layer; aferromagnetic free layer comprising;  a third ferromagnetic layer; and a fourth ferromagnetic layer of high electrical resistivity materialadjacent to said third ferromagnetic layer, wherein said fourthferromagnetic layer is made of Co—Fe—Hf—O; and a spacer layer ofnonmagnetic electrically conductive material disposed between saidsecond ferromagnetic layer and said third ferromagnetic layer; and aninsulation layer disposed between the second shield layer of the readhead and the first pole piece layer of the write head; an actuator formoving said magnetic read/write head across the magnetic disk so thatthe read/write head may access different regions of the magneticrecording disk; and a recording channel coupled electrically to thewrite head for magnetically recording data on the magnetic recordingdisk and to the SV sensor of the read head for detecting changes inresistance of the SV sensor caused by rotation of the magnetization axisof the AP-coupled free layer relative to the fixed magnetizations of thefirst and second pinned layers in response to magnetic fields from themagnetically recorded data.
 11. The disk drive system as recited inclaim 10 wherein said fourth ferromagnetic layer is made of(Co₉₀—Fe₁₀)₆₀—Hf₁₀—O₃₀.
 12. The disk drive system as recited in claim 10wherein said fourth ferromagnetic layer is made of(Co_(a)—Fe_(b))_(x)—Hf_(y)—O_(z) in atomic percent, where 40%≦x≦70%,5%≦y ≦25%, 20%≦z≦40%, 70%≦a≦95%, 5%≦b≦30%, x+y+z=100% and a+b=100%.